This invention relates to the use of a C3-C5 hydrocarbon such as propane as a chain transfer agent in the preparation of fluoropolymers, especially to the free radical polymerization of vinylidene fluoride monomer optionally conducted in the presence of other fluorinated olefins.
Various efforts have been tried over the years to find a suitable chain transfer agent for such polymerizations, as disclosed in the background section of U.S. patent application Publication 2002/0147289 A1. For example, the emulsion polymerization at moderate pressure of vinylidene fluoride using fluorinated surfactant and, as a free-radical initiator. diisopropyl peroxydicarbonate (hereinafter referred to as IPP) is taught in U.S. Pat. No. 3,475,396. The same patent teaches that the amount of fluorinated surfactant necessary in the system can be reduced if a chain transfer agent is present in the reaction system. U.S. Pat. No. 4,569,978 discloses the use of trichlorofluoromethane (CFC-11) as a chain transfer agent to reduce or eliminate the discoloration and cavity formation phenomenon but this CFC is an ozone depleting material and its use is being, banned worldwide. U.S. Pat. No. 3,635,926 discloses an aqueous process for making tetralluoroethylene/fluorovinyl ether (TFE/FVE) copolymers in the presence of chain transfer agents such as hydrogen and methane in combination with CFCs and HCFCs. In this patent only perfiltoromonomers (mainly TFE) were considered and methane was the most preferred chain transfer agent since it exhibited a reasonable chain transfer activity in the polymerization of perfluoromonomers; however, high alkanes were reported to be too active to be used in polymerization due to undesired (slowing) effect on the polymerization rate.
The aforesaid Application 2002/0147289 discloses the use of ethane as a chain transfer agent in a free radical polymerization of vinylidene fluoride. However, higher alkanes, including propane, were reported to be too active to be used due to an undesired slowing effect on the polymerization rate.
In contrast to above disclosures regarding fluorinated monomers, it has surprisingly been found that the use of C3-C5 hydrocarbons such as propane as a chain transfer agent in the vinylidene fluoride polymerization process results, particularly in the case of vinylidene fluoride homopolymers, in a product with good color which resists discoloration at elevated temperatures. In fact, propane has been found to be about ten times as efficient as CFC-11 and about three times as efficient as ethane. Indeed, among all hydrocarbons, propane surprisingly provides the highest polymerization rate per initiator consumption at a given degree of polymerization. Propane is also inexpensive and non-hazardous.
In a process for the free radical polymerization of vinylidene fluoride monomer, optionally in the presence of other fluorinated olefins, this invention provides the improvement comprising the use of a C3-C5 hydrocarbon such as pentane, butane or, preferably, propane as the chain transfer agent. The amount of hydrocarbon can vary widely, but, in the case of propane, less is required than with previous agents such as ethane. The hydrocarbon can be added in batch or continuous feed, depending on the desired molecular weight distribution. The polymerization media normally comprises water or carbon dioxide (such as supercritical and/or liquid carbon dioxide). The preferred free radical initiators are di-n-propyl peroxydicarbonate or di-isopropyl peroxydicarbonate. In another preferred process aspect, vinylidene fluoride homopolymer is produced.